Conventionally, electrical characteristics of electronic components are measured using an automatic characteristic selector in an electronic-component mass-production process. Since a measuring system of the automatic characteristic selector has a circuit characteristic different from a reference measuring system, it is possible to improve a yield by correcting a value measured by the automatic characteristic selector to estimate a value measured by the reference measuring system. Techniques, such as SOLT calibration, TRL calibration, and RRRR/TRRR calibration, are known as such a correction method.
First, TRL/SOLT calibration will be described.
The TRL calibration is the most effective conventional technique that can be used to measure a true value of a scattering coefficient matrix of a measurement-subject surface mount device. In addition, the SOLT calibration is known as a widely used conventional technique. These will be briefly described.
To obtain a true value of the measurement subject, an error factor of a measuring system has to be identified and an influence of the error factor has to be removed from a measurement result. FIG. 1 shows an error model used in a representative error removal method (calibration method).
More specifically, as shown in FIG. 1(a), a subject electronic component 2 is connected to a transmission line formed on an upper face of a fixture 10. Connectors 51 and 61 provided at one end of coaxial cables 50 an 60 are connected to connectors 11a and 11b provided at respective ends of the transmission line of the fixture 10, respectively. The other ends of the coaxial cables 50 and 60 are connected to measurement ports of a network analyzer, not shown. Arrows 51a and 61s show calibration planes.
FIG. 1(b) shows an error model of the TRL correction. A circuit 52 on one measurement-port side represented by scattering coefficients e00, e01, e10, and e11 and a circuit 62 on the other measurement-port side represented by scattering coefficients f00, f01, f10, and f11, are connected between a circuit 12 of the fixture represented by scattering coefficients S11A, S12A, S21A, and S22A and pairs of terminals a1-b1 and a2-b2.
FIG. 1(c) shows an error model of the SOLT correction. A circuit 54 on one measurement-port side represented by scattering coefficients EDF, ERF, 1, ESF and a circuit 64 on the other measurement-port side represented by scattering coefficients ELF and ETF are connected to respective sides of a circuit 14 of the fixture represented by scattering coefficients S11A, S12A, S21A, and S22A.
To identify an error factor in the SOLT calibration, measurement has to be carried out for at least three kinds of devices having known scattering coefficients mounted on a subject measuring plane. As shown in FIG. 2, traditionally, opened (open), short-circuited (short), and terminated (load=50Ω) standards 80, 81, and 82 are often used. Since it is extremely difficult to realize preferable “opened” and “terminated” standards except for in a coaxial environment, calibration cannot be performed at ends of the fixture 10 (calibration planes shown by the arrows 51s and 61s). Since such standards can be realized by a method, such as sliding load, in the coaxial environment, this method is widely used and is referred to as the SOLT calibration.
The TRL calibration uses, instead of the hard-to-realize device-form standards, port-directly-connected state (through), total reflection (reflection, generally short-circuited), and several kinds of transmission lines (lines) having different length as standards. Since manufacture of the transmission lines of the standards having known scattering coefficients is relatively easy and characteristics of the total reflection can be predicted relatively easily if the total reflection is realized by short-circuit, the TRL calibration is known as the most accurate calibration method used in, particularly, a waveguide environment.
FIG. 3 shows an error-factor determining method of the TRL calibration. In the drawing, shading is attached to transmission lines. As indicated by arrows 2s and 2t, calibration planes are parts connected to devices. To identify an error factor, a board 86 of the port-directly-connected state (through), a board 83 of the total reflection (reflection, generally, short-circuited), and boards 84 and 85 of the several kinds of transmission lines (lines) having different lengths are used as standards. In this example, “through” indicates so-called “zero-through”. At the time of measurement of a subject, measurement is carried out after a subject 2 is series-connected to a measurement board 87, which is extended by the size of the subject.
The abstract of the TRL and SOLT calibration is as described above. These techniques have following two problems.
(1) A calibration error is caused unless all error factors caused at connection parts of coaxial-planer transmission lines are equal in transmission lines (several kinds of lines and reflection) and “through” serving as the standards. Even if the same kind of connectors are used in each of the standards, an influence of a connector characteristic variance becomes significantly large when the error factor of each connector differs, and thus, it is substantially impossible to carry out calibration as the frequency approaches the millimeter waveband.
(2) To solve the above-described problem, available fixtures are improved to avoid the influence of the connector measurement variance by connecting a common coaxial connector to a standard transmission line with a contact. However, it is structurally difficult to guarantee a sufficient pressing load applied to the contact part since a coaxial pin may be damaged, and calibration often becomes unstable due to the unstable contact. In addition, since the transmission lines and the coaxial pin generally become thinner as a measurement frequency becomes higher, a measurement variance increases due to reproducibility of positioning of these elements.
To solve these problems, so-called RRRR/TRRR calibration methods have been proposed.
An abstract of the RRRR/TRRR calibration methods will be described next.
These methods are characterized to identify a measuring system error up to ends of a transmission line by short-circuiting a signal conductor and a ground conductor at several predetermined positions of a single transmission line and to be able to highly accurately measure a high-frequency electrical characteristic of a surface mount device. These methods are advantageously free from the problems regarding the transmission-line characteristic variance and the variance of the connection state of the transmission lines and the coaxial connectors involving the TRL/SOLT calibration methods.
As shown in FIG. 4 and FIG. 5, error models are similar to those of the SOLT/TRL calibration. More specifically, FIG. 4 shows an error model of the TRRR calibration, which is the same as the error model of the SOLT calibration shown in FIG. 1(c). FIG. 5 shows an error model of the RRRR calibration, which is the same as the error model of the TRL calibration shown in FIG. 1(b).
A characteristic of the RRRR/TRRR calibration methods is a method for measuring a “standard measurement value” used in calibration. The SOLT and the TRL use measurement values of a standard device and a standard transmission line as the “standard measurement value”, respectively. As shown in FIG. 6, the RRRR/TRRR calibration methods use values measured by changing a short-circuited reference position on a measurement board 10a as the “standard measurement value”. Since an influence of connectors is not caused, the RRRR/TRRR calibration methods are more accurate and more effective than the SOLT calibration and the TRL calibration in desktop measurement.
However, since the TRRR/RRRR calibration uses a change in reflection coefficients resulting from different connection positions of a short-circuited reference (short chip 2s) to fixture transmission lines 10s and 10t as the calibration reference, the connection position of the short-circuited reference has to be significantly changed when a wavelength of a signal to be measured is long (frequency is low) and, thus, T1 and T2 shown in the drawing have to be extended. Accordingly, a length of the measurement board 10a (size in a direction indicated by an arrow L) has to be extended. In addition, since an automatic characteristic selector used in a mass-production process has a restriction regarding the structure and size, it is difficult to provide a correction ground terminal in the fixture 10a and to configure the short chip 2s so that the position thereof can be determined accurately (see, for example, Patent Documents 1 and 2).    Patent Document 1: WO2005/101033    Patent Document 2: WO2005/101034    Non-Patent Document 1: Application Note 1287-9: In-Fixture Measurements Using Vector Network Analyzers, (© 1999 Hewlett-Packard Company)